Controlling the stimulation implies making permanent an adjustment of various parameters such as the frequency of stimulation, the atrio-ventricular delay (AVD), and the inter-ventricular delay in the case of a biventricular stimulation. These various parameters are adjusted according to signals delivered by a sensor, for example, a sensor of the ventilation-minute (MV), which is a physiological parameter used as a factor representative of the instantaneous metabolic needs (also referred to as cardiac output requirements) of the patient. Sensors that measure a physical characteristic such as movement via piezoelectric transducers, also are known.
Another factor that can be desirable to know is the cardiac flow. This is because it is possible, particularly with a multisite pacemaker, to obtain an indication of the cardiac flow, and thus of the fraction of ejection. The latter is the hemodynamic reference parameter that is used to optimize the stimulation on the various sites.
The published EP-A-1 116 497 and its corresponding U.S. Pat. No. 6,604,002 B1, commonly assigned herewith to ELA Médical, describes a manner of taking a dynamic measurement of bio-impedance allowing for an evaluation of diastolic and systolic volumes, and thus obtaining an indication of the cardiac flow and the ejection fraction. The signal obtained can be used to control (i.e., modify) the heart rate and/or the atrio-ventricular delay in a direction that will obtain a maximization of the cardiac flow. It is also possible to use the parameter thus measured to control the interventricular delay, in the case of a biventricular stimulation.
In addition, and more particularly, this prior art document describes a measurement technique for measuring the bio transvalvular impedance (i.e., the impedance between the atrium and the ventricle located on the same side of the heart) by a tripolar configuration, with injection of a current pulse between an atrial site and a ventricular site, and the collection (detection) of a differential potential between an atrial site and a ventricular site, with one of the sites being common to the injection and the collection, and with one site being specific for the injection and one site specific for the collection. The injected current pulse is a current of low amplitude, that is an amplitude that is insufficient to excite the cardiac cells or initiate a depolorization.
The published EP-A-1 138 346 and its corresponding U.S. Published Patent Application No. US 2001-0034540 A1 (published on 29 Oct. 2001), now U.S. Pat. No. 6,725,091, also commonly assigned herewith to ELA Medical, describes another type of measurement of the bio impedance, namely, a trans-septum bio-impedance, i.e., between one site located on one side of the heart and a second site located on the other side of the heart. Through this technique, one can obtain a value representative of the ejection fraction, although the bio-impedance signal is weaker than in the case of the transvalvular bio-impedance measurement, and also is more influenced by the impedance of the septum tissues.